1. National Institute of Chemistry, P.O. Box 3430, 1001 Ljubljana, Slovenia and Department of Mathematics and Computer Science, Drake University, Des Moines, Iowa 50311, The Rud-er Bošković Institute, P.O.B. 180 HR-10002 Zagreb, Croatia, and Natural Resources Research Institute, University of Minnesota at Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811